This research project is an experimental study of the synaptic organization of the vertebrate retina. Although the receptive field properties of ganglion cells are quite pertinent to visual function, little is known concerning the presynaptic pathways which underlie the properties of these cells. This research proposal will examine the influence of horizontal cells upon the receptive field properties of ganglion cells in the rabbit retina and will investigate whether this horizontal cell to ganglion cell pathway is modulated by light level and by synaptic input onto horizontal cells in the rabbit and fish retinas. Horizontal cells will be polarized by intracellular current injections while simultaneously recording from nearby single-unit ganglion cell spike activity. These experiments will determine whether horizontal cells contribute to ganglion cell receptive field properties in addition to their surrounds, and whether horizontal cells influence some, but not all, of the ganglion cell classes of the rabbit retina. Because different types of rabbit horizontal cells receive different proportions of rod and cone input, polarization of these cell types, which will be identified by dye injection, will help reveal whether rod and cone pathways exert different effects upon ganglion cell receptive field properties. Transmitter antagonists will also be applied during current injections into horizontal cells to determine whether drugs with known actions can alter the effects of current injection on ganglion cell discharges. These latter data may elucidate circuitry of the outer retina. Additional studies on the carp and rabbit will investigate the possible modulation of horizontal cell receptive field size and responsiveness and ganglion cell surround size and sensitivity by prolonged dark adaptation, dopamine and cyclic AMP. Knowledge of neural circuitry and transmitter function in mammalian retina will increase understanding of human retinal processes, enhance the diagnosis of retinal pathologies and provide the basis for drug therapy for retinal disorders. The identification of specific influences of dopaminergic neurons in the retina will further our understanding of visual pathologies associated with Parkinson's disease.